The catalyst system disclosed in U.S. Pat. No. 4,302,565 includes a precursor containing magnesium, titanium, a halogen, and an electron donor, and is found to be quite useful in the production of polymers, which are advantageously used in injection molding. These polymers are characterized by relatively high densities, low melt flow ratios, and narrow molecular weight distributions, and the catalyst system is characterized by high activity. Melt flow ratio is the ratio of flow index to melt index. Melt index is determined under ASTM D-1238, Condition E. It is measured at 190.degree. C. and reported as grams per 10 minutes. Flow index is determined under ASTM D-1238, Condition F. It is measured at 10 times the weight used in the melt index text above and reported in the same manner, i.e., grams per 10 minutes.
While the characteristics of the catalyst system and the polymers produced using same are of commercial significance, there is a need for a catalyst system which can provide polymers having still lower melt flow ratios and, when molded or extruded, improved physical properties including higher dart drop, lower hexane extractables, and high impact strength, without sacrifice in the high activity aspect of the catalyst system or even with some improvement.
It was found that lower melt flow ratios could be achieved, for example, by substituting trimethylaluminum for triethylaluminum as a catalyst activator or modifier, but there was a loss in catalyst activity, and the trimethylaluminum was very expensive when compared to triethylaluminum and highly pyrophoric. Other attempts at achieving lower melt flow ratios were similarly frustrated.